How do transdermal skin patches work?

Years ago the skin was considered a barrier. You weren’t at risk of hurting your body unless something punctured the skin and created a wound. Today we know better. The skin is not necessarily a barrier against invaders, but more of a permeable layer through which chemicals pass directly into our bodies.

This is how a nicotine, hormonal or morphine patch administers medication. And yet, if medication can be administered transdermally, then isn’t everything else we put next to our skin getting in as well? The answer is, it's not that simple.

Skin basics

Any talk about transdermal absorption requires at least a fundamental understanding of the nature of our skin. It is made up of several layers: the epidermis (the part we can touch), an outermost waterproof layer; the dermis (where blood vessels live); and subcutaneous tissue called the hypodermis. A medication patch adheres to the skin and delivers medication through the bloodstream. This is made possible in some cases when drugs are combined with substances like alcohol to increase their ability to penetrate the skin. The molecules of the medication also must be small enough to penetrate. However, not all drugs can be administered this way because the barrier properties of the skin still provide a significant challenge.

Substances like medication patches must remain in contact with the skin for quite a while to be absorbed. Likewise, repeated applications of lotion or body oil may absorb as well. This becomes a problem with many body products that contain ingredients like synthetic fragrance and preservatives, which we would rather not be absorbed into our bloodstream. Interestingly, the longer that skin is exposed to water, the more permeable it becomes. When fingers prune after a long soak in the tub, that’s because water has seeped through its many layers. Studies show hydrated skin is 3.3 times more likely to absorb substances.

In the case of synthetic fragrance, which has been shown to disrupt normal hormonal function, we can assume that a certain percentage of these molecules become absorbed through the skin. However, not all ingredients are capable of getting in because they may be too heavy, their molecular weight too large to penetrate.

The Environmental Working Group (EWG) says epidemiological studies have linked high phthalate levels, a chemical used in a wide variety of consumer products such as fragrances, cosmetics, shampoos, laundry detergents, soft toys and children’s clothing, to reduced sperm motility and concentration, increased damage to sperm DNA, and alterations in hormone levels in adult men.

Results from one study suggest that breakdown products of one particular phthalate, DEHP, may alter thyroid hormone levels. In another study, increased levels of certain phthalates increased waist circumference and insulin resistance, leading to Type 2 diabetes.

Keep it clean

If you are trying to maintain a toxin-free zone, it may be wise to choose natural and organic products whenever possible including cosmetics, hair, nail, and body products, makeup and laundry detergents since they will have direct contact with skin. The EWG says to skip products that use the term “fragrance” in the list of ingredients and instead opt for those that list each fragrance ingredient. Many common ingredients can contain impurities linked to cancer and other health concerns. Avoid these common ones when possible:

DMDM hydantoin

Diazolidinyl urea

Imidazolidinyl urea

Ceteareth

Polyethylene glycol and PEG

The EWG is creating an app to complement its Skin Deep Cosmetics Database, which rates products based on known hazards associated with ingredients listed on labels. The app will include a barcode reader to look up products at the store and tips for avoiding chemicals when as they relate to the skin.